1. Field of the Invention
The present invention relates to an ultrasonic doppler diagnostic system for obtaining blood flow information, within the human body under examination, utilizing an ultrasonic Doppler effect, and more specifically, to an ultrasonic doppler diagnostic system capable of detecting low velocity blood flow component information separately from clutter component information.
2. Description of the Related Art
There has been used an ultrasonic diagnostic system for,transmitting ultrasonic beams within the human body and receiving the ultrasounds reflected by tissue in the human body, thus diagnosing diseases of the viscera and the like. In one aspect of this ultrasonic diagnostic system, or in an optional function of an ultrasonic diagnostic system for displaying a tomographic image (B-mode), there has been used an ultrasonic diagnostic system for receiving ultrasounds reflected by blood cells flowing within the human body thereby obtaining blood flow information such as velocity, variance, power and the like of the blood flow.
FIG. 10 is a schematic construction view of one example of a conventional ultrasonic diagnostic system.
A transmission control section 11 supplies pulse signals Tp to a number of transducers (not shown) constituting an ultrasonic probe 12 in each specified timing. Thus each transducer transmits ultrasonic pulse beams within the human body under examination (not shown). For example, in sector scanning, a specified number (e.g. eight pulses) of ultrasonic pulse beams are emitted along a given direction. The ultrasonic pulse beams are reflected by blood cells flowing within the human body under examination or the other tissues, and are received by each transducer in the ultrasonic probe 12. The received signals Ap, received by each transducer, are input to a beamformer 13 and are beamformed therein so as to satisfy reception dynamic focussing. The received signal S thus obtained is input in a B-mode image detecting section 14 and a blood flow information detecting section 15.
The B-mode image detecting section 14 generates a signal S.sub.A for carrying tomographic image (B-mode) display on the basis of the input received signal S. The signal S.sub.A is supplied to a display section 16 composed of a CRT display or the like, thus displaying a tomographic image for diagnosis.
Meanwhile, the blood flow information detecting section 15 detects blood flow information on the basis of the input received signal S utilizing a Doppler effect as described below.
Namely, the ultrasounds reflected by blood cells within the blood flow are subjected to frequency shift by the movement of the blood cells. The frequency shift amount (Doppler shift frequency) f.sub.d is represented as follows: EQU f.sub.d =(2V cos.theta./C).multidot.f.sub.c ( 1)
where V is the blood flow velocity, .theta. is an angle formed by the two intersecting directions of the blood flow and transmitted ultrasonic beam, f.sub.c is a center frequency of the transmitted ultrasounds and C is a velocity of ultrasounds transmitted within the human body.
Further, the center frequency f of the received signal receiving the reflected ultrasounds is represented as follows: EQU f=f.sub.c +f.sub.d ( 2)
Accordingly, by detection of the Doppler shift frequency f.sub.d and also detection of a blood vessel extending direction on the basis of the signal SA for carrying the above tomographic image, the blood velocity V can be detected. The Doppler shift frequency fd can be obtained using a wide variety of methods, for example, an auto-correlation method, FFT method, and micro-displacement measuring method [a cross-correlation method (Yagi, et al, "Micro-displacement measurement for inhomogeneous tissue utilizing the spatial correlation of analysis signal", pp. 358-360, Literature of No. 54 Meeting of Japan Ultrasonic Medical Institute); a phase tracking method (Araki, et al, "Tissue displacement measurement in living subject by phase tracking processing", pp. 445-446, Literature of No. 57 Meeting of Japan Ultrasonic Medical Institute, and "Ultrasonic Diagnostic System", Japanese Patent Application No. hei 2-273910, Application Date: Oct. 12, 1989); and a method oriented to observation data (Yamagosi, et al, "Estimation method for micro-displacement in a reflected type independently from the random structure of scatterer", pp. 233-234, Literature of No. 56 Meeting of Japan Ultrasonic Medical Institute, and "Ultrasonic diagnostic system", Japanese Patent Application No hei 2-088553, Application Date: Apr. 3, 1989)].
The signal S.sub.B carrying blood flow information thus obtained is input to the display section 16 and is, for example, superposed on the above tomographic image, so that the blood flows in the directions of approaching to and separating from the ultrasonic probe are displayed, for example, as red and blue, respectively.
Hereafter, the discussion will be directed to Doppler detection by quadrature detection and MTI filtering, which is per se conventional but is modified advantageously in each of the inventive embodiments of FIGS. 2, 4, and 8.
FIG. 11 is a block diagram of one example of a conventional ultrasonic diagnostic system showing a portion equivalent to a blood flow information detecting section as shown in FIG. 10. As shown, the received signal S output from the beamformer is input to a quadrature detector 17 to be detected by 90 degrees. FIG. 12 is a block diagram showing an internal construction of the quadrature detector 17.
The received signal S input to the quadrature detector 17 is divided into two lines, which are respectively input to multipliers 17a and 17b. Meanwhile, the multipliers 17a and 17b receive two sinusoidal signals (carrier signals) SIN, COS different in phase by 90 degrees from each other. The multipliers 17a and 17b multiply the received signal S by the carrier signal SIN, and the received signal S by the carrier signal COS respectively, thus generating two signals SM.sub.I and SM.sub.Q, each having both frequencies of addition and difference of the two signals prior to multiplication. These signals SM.sub.I and SM.sub.Q are made to pass through low-pass filters 17c and 17d respectively. This generates an I component SM.sub.I and Q component SM.sub.Q of the received signal S after 90-degree phase detection, each carrying only a signal having the frequency of the difference between the two signals mentioned above, which are output from the quadrature detector 17. The I component SM.sub.I and Q component SM.sub.Q of the received signal S thus output from the quadrature detector 17 are input to respective A/D converters 18a and 18b to be A/D converted, and are then input to respective Moving Target Indicator (MTI) filters 19a and 19b. Each of the MTI filters 19a and 19b is a digital filter for cutting off a low frequency signal, similarly to that used in a radar, and is widely used in the field of the ultrasonic diagnostic systems. It is generally composed of a delay circuit providing a delay time equivalent to the repeated cycle of the pulse signals and integral/adding device. The MTI filters 19a and 19b are used to remove clutter component information. In general, the received signal S includes not only blood flow information but also relatively slow clutter component information mixed as high noise. More specifically, the clutter component is due to the motion of the human body, under examination, other than the blood flow and consequently has a power 100 times as much as the blood flow component. The I component SC.sub.I and Q component SC.sub.Q of a clutter removing signal SC output from the MTI filters 19a and 19b are input in an auto-correlator 20. The auto-correlator 20 generates, by auto-correlation, a signal S.sub.B carrying the blood flow information such as velocity, velocity distribution, power and the like of the blood flow.
FIG. 13a and 13b are views showing the characteristic of the MTI filter.
The abscissa represents a Doppler shift frequency f.sub.d (refer to the equation (1)). Further, a polygonal line 31 represents the characteristic of the MTI filter. The MTI filter has such a characteristic as to cut off the signals within a frequency band of .vertline.fd.vertline..ltoreq.Th with f.sub.d =0 taken as the center and to pass the signals within a frequency band of .vertline.fd.vertline.&gt;Th. Further, crests 32 and 33 represent the Doppler shift frequency distribution of the clutter components and the blood flow carried by the received signal S.
As shown in FIG. 13a, in the case that the blood flow velocity is high and the crest 33 is greatly separated from the crest 32, by determining the signal elimination band of the MTI filter to cut off the clutter component information corresponding to the crest 32 and to pass the blood flow information corresponding to the crest 33, the clutter removing signal SC (SC.sub.I, SC.sub.Q) from which the clutter component information is selectively removed, can be output. Meanwhile, in the case that the blood flow velocity is very slow and the crest 33 comes closer to the crest 32, as shown in FIG. 13b, there occurs such a problem that, by determining the characteristic of the MTI filter to remove the clutter information, the blood flow component information is also removed, even if the crest 32 is separated from the crest 33 as yet. Accordingly, only the clutter component information cannot be selectively removed. In particular, there has been enhanced the requirement for detecting the blood flow information of the abdomen portion such as the liver, and therefore, it has become important to detect the very slow blood flow having a Doppler shift frequency similar to that of the clutter components.
There has been disclosed such an attempt as to obtain the signal carrying only the blood flow component separately from the clutter components, in Japanese Patent Laid-open sho No. 63-59938, No. hei 1-110351, No. hei 1-314552 and No. hei 2-167151.
In Japanese Patent Laid-open No. sho 63-59938, there is provided a clutter map on the basis of intensity of the reflected ultrasounds and the order of the MTI filter is switched by the clutter map. This technique determines the level of removing the clutter components according to intensity of the clutter components, and is not intended to effectively remove the clutter component information even if the Doppler shift frequency distributions of the clutter components and blood flow component become closer to each other.
In Japanese Patent Laid-open No. hei 1-1103351, there is proposed a technique of extracting the carrier signals SIN and COS as shown in FIG. 12 from the clutter component signal themselves thereby effectively removing the clutter component information. It may be considered that if the carrier signals SIN and COS are correctly extracted, the clutter component information can be effectively removed; however, by the proposed technique, it may be difficult to correctly extract the carrier signals SIN and COS.
The technique disclosed in Japanese Patent Laid-open No. hei 1-314552 is intended to correct the signal including the clutter component information and blood flow information, and accordingly cannot effectively remove only the clutter component information separately from the blood flow information.
Further, in Japanese Patent Laid-open No. sho hei 2-167151, there is proposed a technique of determining the characteristic of the MTI filter for removing the clutter components on the basis of a point Doppler signal. However, the technique complicates the construction because of obtaining the point Doppler signal, and cannot obtain the information per one point due to the point Doppler. Also, in the case of the Doppler shift frequencies of the clutter components and blood flow as shown in FIG. 13b, only the clutter component information cannot be effectively removed.